Molecular simulation keeps oil flowing at home and abroad

Hydrates can form solid masses which completely block transport pipes.

Hydrates are a big problem in the oil and gas processing industries. Hydrates are ice-like crystalline materials which make use the unusual environment within a pipe (high pressure and low temperature) to form on the inside of pipes used to carry hydrocarbons. They are usually composed of water molecules and ‘guest’ vapour, such as methane. These hydrate solids grow to such a size that they impede and ultimately block the flow of oil or LNG.

A group working at Texas A&M University at Qatar (TAMUQ) and led by Dr. Ioannis Economou, has gone back to basics to try and solve this problem, by using computational molecular dynamics. Theoretical concepts are applied to various molecules and mixtures of molecules, then supercomputers predict how these molecules and mixtures may perform at various temperatures and pressures within a pipe.

These simulations are over a timescale of hundreds of nanoseconds, which is significant at a molecular level, and can only be performed thanks to parallel processing on the high-performance computers at TAMUQ.

They found that you can prevent this formation of hydrates by knowing the temperature and pressure at which the hydrates form or melt. So if your process allows you to operate outside of this range of conditions, then the hydrates will not form. If you operate at a higher temperature, they won’t form, but if the temperature drops and the pressure increases, then they will form. Having the data and predictions allow you to know at which conditions you should operate. Not only is this useful in the hot regions of Qatar and the Middle East, it has immense practical applications in the colder conditions of the oil and gas industry of Alaska.

Hydrates with slightly different geometries can form massive structures.

So far the project has examined methane hydrates, but the next step will also look at CO2 hydrates. This is important in other ways since the increase in temperature of the world’s oceans means that hydrates deep underwater are now starting to melt and release methane in the atmosphere. The gas released from the oceans could exacerbate global warming and either needs to be stopped, slowed, or by some method we capture the gas which is being released. Methane is a short-lived, but intensely active greenhouse gas, which alongside CO2 is contributing to global temperature increases.

The new project, using metal-organic frameworks known as zeolites will allow for the energy-efficient, environmentally friendly separation of gas mixtures and could be an important step in dealing with this additional new source of greenhouse gases.

The lattice structures can be modelled, and
the conditions under which they melt can
be predicted.

The wide-ranging work has been performed at TAMUQ in collaboration with Demokritos, the Greek national research center. The theory and simulations are carried out in Qatar, and an experimental facility in Greece allows real-world experimentation and measurements. Such collaborations are essential for the project and one of the strengths of the QNRF approach to funding, said Dr Economou.

“What I really like about QNRF is it gives us opportunities to collaborate with institutions outside Qatar. For example, we don’t have an experimental facility here, but we can do that work in Demokritos, and we get more work done in a fraction of the time compared to if I had to build an experimental facility here. QNRF also give us the opportunity to exchange information. One co-author is based in Greece, but spends 2-3 months each year in Qatar, in my group, as a visiting scientist, so we gain a lot from him. One post-doc spent last summer in Greece working on the experimental set-up, so there is a very efficient transfer of knowledge between the groups, which allows us to get many things done.”

The paper entitled: "Prediction of the Phase Equilibria of Methane Hydrates Using the Direct Phase Coexistence Methodology" was selected as the featured article of the 28 January 2015 issue of the Journal of Chemical Physics and also featured on the cover page. Seehttp://scitation.aip.org/content/aip/journal/jcp/142/4

NPRP 6-1547-2-632
Gas Storage and Transportation, and Separation Process Development based on Hydrates